He's trying to create an ePub for the Aldiko (which I'm assuming is an ereader) using an image as cover. Creation works using Calibre, the cover is displayed in the "book shelf" (it's unclear whether this means the Calibre library or the Aldiko), but when he opens the book, no cover is displayed. He's asking whether Aldiko expects something special of if he's doing something fundamentally wrong.

To the original poster: As I've already implied, there is some information missing in your description. Does Calibre show the correct cover in the library? What format is the cover? (Though, since the creation/conversion goes flawlessly, I'm assuming it's at least something allowed in an ePub.) Does the Calibre reader show the cover correctly?

And yes, you really should use the correct language in the respective forums, that makes it a lot easier for all of us. There's a german forum here for a reason.